1. Field of the Invention
The present invention relates to a magnetic agitator for fluids, in particular for agitating sterile fluids and a method for operating such an agitator.
2. Discussion of Related Art
It is hard to imagine process engineering, particularly in the biotech sector, the pharmaceutical industry, and the food industry, without mechanically agitated reactors. The problematic seal of the agitator shaft has led, for reasons of both sterility and biosafety, to a preference for floating agitator bearings in lieu of packing glands, lip seals, or sliding ring seals. Magnetically coupled agitator systems in which the separating vessel is part of a welding flange in the container wall, thus guaranteeing a hermetic separation of the inside of the container from the surrounding atmosphere, have been disclosed by the applicant and been successfully used in practice.
U.S. Pat. No. 4,993,841 and European Reference EP-A1-0,399,972 disclose magnetic agitators equipped with impellers embodied so that they can be driven in a contactless, inductive, or magnetic fashion. These impellers have a multitude of agitating blades extending radially outward from them that function as agitating tools or agitating elements. During operation, a vortical flow is generated in the container, with the impeller as the center point; the countervailing force of the impeller pulls upward and thus relieves the pressure on the bearing of the impeller. The suction in the direction of the impeller, however, is undesirable since it encourages the entrainment of gas into the agitated fluid and with some fluids, generates foam.
German Reference DE-A1-42 32 934 discloses a double impeller for bottom-mounted drive units designed to agitate sterile fluids, which has an improved mixing and agitating action. The agitating tools on the impeller head are embodied in the form of propellers. This generates an upwardly directed flow, thus preventing an undesirable entrainment of gas and generation of foam. The impeller head is provided with an opening at the bottom to accommodate the separating vessel in a central cavity. The step of providing at least one connecting line between the central cavity and the outer surface serves to continuously flush the cavity of the impeller with fluid, thus keeping the impeller clean.
On the underside of the impeller, additional flow-generating surfaces are provided, which are able to generate a downward or lateral flow directed counter to the flow generated by the propeller. These flow-generating surfaces are preferably embodied in the form of lateral wings or turbine-like lower end surfaces. The flow generated by the lower flow-generating surfaces partially compensates for the axial, downwardly directed forces of the propeller. This should reduce the pressure on the product-lubricated bearings, thus extending their expected service life. In order to actually achieve this effect, however, it is necessary to take into account the fact that:    i) a second flow is generated in the container that negatively influences the desired flow dynamics;    ii) the actual agitating elements are not easily interchangeable because depending on their weight and the flow generated, it is also necessary to adapt the countervailing force-generating bottom surfaces in a corresponding fashion;    iii) at a low fluid level in the container, only the lower surfaces extend down into the fluid and in the absence of the countervailing force from above, the agitator can be driven upward, away from the bearing pin;    iv) with tall and heavy agitating elements, for example in bottom agitation units equipped with a number of segmented agitator disks, the lower flow-generating surfaces are very large in area.